This invention relates to the manufacture of drawn sheet glass and provides a process and plant for the production of such glass. The invention extends to sheet glass produced using such a process or plant.
In the continuous production of drawn sheet glass it is necessary to form molten glass into a ribbon and then to anneal it as it cools in order to form a cool, stress-free ribbon of glass which can then be cut into sheets.
Classical processes are known in which the ribbon is cooled in a vertical annealing lehr and others are known which make use of a horizontal annealing lehr.
These processes, often referred to as "vertical drawing" and "horizontal drawing" processes, are of quite different types. In the case of a vertical annealing lehr, the ribbon is drawn upwardly by rollers, through the annealing lehr which may be 6 m to 10 m in height depending on the actual process used, and into a cutting loft above the lehr where the ribbon is cut into sheets. The lowest rollers are located a sufficient distance above the root of the ribbon (usually 2 m or more) for the glass to be hard enough not to be marred by the rollers.
In the case of a horizontal annealing lehr, the upwardly drawn ribbon is folded about a horizontal bending roll onto a horizontal conveyor leading through the lehr. The bending roll is located at a distance above the root of the ribbon (usually 1 meter or so) where the glass is soft enough to be passed to the horizontal lehr. Horizontal annealing lehrs are often more than 50 m long.
The present invention is concerned with a sheet glass manufacturing process wherein molten glass formed in a glass-melting tank flows continuously to a drawing tank whence the glass is drawn upwardly in the form of a continuous ribbon, is folded about a bending roll and then passed to a horizontal annealing lehr.
Advantages of such horizontal drawing processes over vertical drawing processes are well known. The drawn ribbon can be supported and thus stabilized by a bending roll at an earlier stage than by gripping rollers. Because a longer lehr can be used, the ribbon can be cooled more slowly so that it is relatively stress free, and so that the glass is easier to cut. It is also easier to control annealing in a horizontal lehr than in a vertical lehr where convection currents can be particularly troublesome. There is thus less risk that a ribbon will break due to thermal shock in a horizontal lehr. Even if a horizontally drawn ribbon should break, no longitudinal fracture can propagate back beyond the bending roll where the glass is still soft enough to be folded. There is accordingly a greatly reduced risk that any broken glass will fall down into the drawing tank.
Horizontal drawing of glass does however have limitations in the attainable yield of glass of an acceptable standard as regards its uniformity of thickness. This is especially true when drawing very thin glass. In fact for large scale continuous production the practical lower limit of thickness which is observed in any of the known horizontal drawing processes is about 0.6 mm.